The present invention relates to a multilayered gas sensor and a gas concentration detecting system using this sensor.
The multilayered gas sensor has a sensing element which is maintained at an appropriate activated condition by controlling an element temperature (i.e., the temperature of a sensing element) to a predetermined temperature region.
To this end, the element temperature is periodically monitored and the electric power supplied to a heater is controlled to maintain the element temperature to a target value.
In general, the element temperature is indirectly monitored based on an element impedance (i.e., element resistance) known from a relationship between a voltage applied to the sensing element and an obtained sensor current.
FIG. 10 shows the temperature characteristics of a sensing element, according to which the element impedance increases with decreasing element temperature.
In general, the element impedance includes a resistance component of a solid electrolytic substrate and a resistance component of an electric lead portion. The solid electrolytic substrate has negative temperature characteristics according to which the resistance of the solid electrolytic substrate decreases with increasing temperature as indicated by a line {circle around (1)} shown in FIG. 17. On the contrary, the lead portion has positive temperature characteristics according to which the resistance of the lead portion increases with increasing temperature as indicated by a line {circle around (2)} shown in FIG. 17. In FIG. 17, a line {circle around (3)} represents the change of element impedance (ZAC).
The element impedance is inherently a resistance component of the solid electrolytic substrate. However, the actually detected element impedance of a sensor includes a resistance component of the lead portion whose temperature characteristics is opposite to that of the solid electrolytic substrate. Especially, when the gas sensor is in an activated condition (i.e., in a higher temperature region), the percentage of the lead resistance becomes large. This leads to deterioration in the sensitivity of element impedance.
In view of the above-described problems, the present invention has an object to provide a multilayered gas sensor capable of accurately detecting an element impedance in the entire operating region of this sensor. Furthermore, the present invention provides a gas concentration detecting system capable of improving the temperature controllability by the use of the multilayered gas sensor of the present invention.
In order to accomplish the above and other related objects, the present invention provides a first multilayered gas sensor comprising a solid electrolytic substrate having oxygen ion conductivity, a measured gas side electrode provided on one surface of the solid electrolytic substrate, a reference gas side electrode provided on an opposite surface of the solid electrolytic substrate so as to be exposed to a reference gas stored in a reference gas chamber, a first lead having one end connected to the measured gas side electrode and the other end connected to a first signal output terminal, and a second lead having one end connected to the reference gas side electrode and the other end connected to a second signal output terminal. The first multilayered gas sensor is characterized in that the following relationship is satisfied
B/A less than 0.5
wherein xe2x80x98Axe2x80x99 represents an overall resistance value of an electric path including the solid electrolytic substrate, the electrodes, and the first and second leads in a sensor activated condition, while xe2x80x98Bxe2x80x99 represents a resistance value of the first and second leads at a room temperature.
According to a preferred embodiment of the present invention, the overall resistance value xe2x80x98Axe2x80x99 is a target resistance value for a sensor activation control (i.e., an impedance control).
In short, a ratio of the lead resistance value xe2x80x98Bxe2x80x99 to the overall resistance value xe2x80x98Axe2x80x99 is restricted to be less than 0.5. In other words, according to the first multilayered gas sensor, the percentage of the lead resistance with respect to the overall resistance can be restricted to a predetermined smaller value so as to adequately maintain or improve the sensitivity of element impedance. In other words, it becomes possible to enhance the correlation between the solid electrolytic resistance and the overall resistance. If required to assure more accuracy for the detection of element impedance, it will be preferable to restrict the ratio B/A to a more smaller value equivalent to 0.3 or less.
Practically, reducing the percentage of the lead resistance is feasible by reducing a resistance value of the lead portion. For example, it is preferable that at least one of the first and second leads has a lateral cross section equivalent to xc2xd to 5 times a lateral cross section of a corresponding electrode. It is also preferable that at least one of the first and second leads is thicker than the corresponding electrode. It is also preferable that at least one of the first and second leads is wider than the corresponding electrode.
Alternatively, reducing the percentage of the lead resistance is feasible by increasing a resistance value of the solid electrolytic substrate, although the time required to reach a sensor activated condition increases.
According to a preferable embodiment of the present invention, the first and second signal output terminals are provided at intermediate portions of the solid electrolytic substrate. This arrangement is advantageous to reduce the length of a lead connecting the sensor electrode (i.e., the measured gas side electrode or the reference gas side electrode) to its signal output terminal. As a result, the resistance value of the lead portion can be reduced.
Furthermore, it is preferable that the first and second leads contain a ceramic material to improve the adhesion properties and an additive amount of the ceramic material in at least one of the first and second leads is less than or equal to 12.5 wt %.
It is also preferable that at least one of the first and second leads is an electric conductive member having a resistance temperature coefficient less than or equal to 3xc3x9710xe2x88x923/xc2x0 C. If required to assure more excellent performance, it will be preferable that the electric conductive member has a resistance temperature coefficient less than or equal to 2.5xc3x9710xe2x88x923/xc2x0 C.
According to a preferable embodiment of the present invention, the electrodes are bonded on the surfaces of the solid electrolytic substrate and an insulating layer having a low thermal conductivity is provided to isolate the first and second leads from the solid electrolytic substrate.
In a sensor activated condition, the solid electrolytic substrate has a higher temperature. As understood from the characteristics shown in FIG. 17, the adverse influence of lead resistance increases when the temperature is high. In this respect, providing the insulating layer having a low thermal conductivity makes it possible to effectively prevent the temperature of the lead portions from increasing excessively. As a result, it becomes possible to improve the temperature characteristics of the sensor.
The gas sensor is generally equipped with a heater to increase the temperature of each electrode. However, the provision of a heater causes a temperature distribution in the gas sensing element in such a manner the temperature is high in the vicinity of the electrodes compared with the signal output terminals and their vicinities. Considering such temperature distribution, it is effective to reduce the resistance value of a limited lead portion closer to the electrodes.
In view of the above, the present invention provides a second multilayered gas sensor comprising a solid electrolytic substrate having oxygen ion conductivity, a measured gas side electrode provided on one surface of the solid electrolytic substrate, a reference gas side electrode provided on an opposite surface of the solid electrolytic substrate so as to be exposed to a reference gas stored in a reference gas chamber, a first lead having one end connected to the measured gas side electrode and the other end connected to a first signal output terminal, a second lead having one end connected to the reference gas side electrode and the other end connected to a second signal output terminal, and a heater for heating the electrodes. The second multilayered gas is characterized in that at least one of the first and second leads has a low resistance portion located in the vicinity of the electrodes and a high resistance portion located in the vicinity of the signal output terminals.
This arrangement makes it possible to selectively or effectively reduce the resistance value of a lead portion located closely to the electrodes. In other words, according to the second multilayered gas sensor, the percentage of the lead resistance with respect to the overall resistance can be restricted to a predetermined smaller value so as to adequately maintain the sensitivity of element impedance. As a result, it becomes possible to improve the temperature characteristics of the sensor. In other words, a detected element impedance explicitly reflects a resistance change of the solid electrolytic substrate.
According to a preferable embodiment of the present invention, a lateral cross section of the high resistance portion is smaller than that of the low resistance portion. This makes it possible to reduce the cost of the lead portions which are usually a platinum or other noble metallic member.
The present invention provides a third multilayered gas sensor comprising a solid electrolytic substrate having oxygen ion conductivity, a measured gas side electrode provided on one surface of the solid electrolytic substrate, a reference gas side electrode provided on an opposite surface of the solid electrolytic substrate so as to be exposed to a reference gas stored in a reference gas chamber, a first lead having one end connected to the measured gas side electrode and the other end connected to a first signal output terminal, a second lead having one end connected to the reference gas side electrode and the other end connected to a second signal output terminal, and a heater for heating the electrodes. The third multilayered gas sensor is characterized in that at least one of the first and second leads is configured in such a manner that a resistance value per unit length is smaller at a portion near the electrodes and is larger at a portion far from the signal output terminals.
This arrangement makes it possible to selectively or effectively reduce the resistance value of a lead portion located closely to the electrodes. In other words, according to the third multilayered gas sensor, the percentage of the lead resistance with respect to the overall resistance can be restricted to a predetermined smaller value so as to adequately maintain or improve the sensitivity of element impedance. As a result, it becomes possible to improve the temperature characteristics of the sensor. In other words, a detected element impedance explicitly reflects a resistance change of the solid electrolytic substrate.
According to the preferable embodiment of the present invention, the third multilayered gas sensor satisfies the following relationship
B/A less than 0.5
wherein xe2x80x98Axe2x80x99 represents an overall resistance value of an electric path including the solid electrolytic substrate, the electrodes, and the first and second leads in a sensor activated condition, while xe2x80x98Bxe2x80x99 represents a resistance value of the first and second leads at a room temperature.
In this case, the overall resistance value xe2x80x98Axe2x80x99 is a target resistance value for a sensor activation control (i.e., an impedance control).
Moreover, it is preferable that first to third multilayered gas sensor of the present invention further comprise a resistance detecting means for detecting a resistance value of the solid electrolytic substrate based on electric signals obtained from the signal output terminals, and a heater control means for controlling electric power supplied to a heater based on the resistance value detected by the resistance detecting means.